The invention relates to a braking system for a utility vehicle which can be coupled pneumatically to a trailer, having at least one electronic control unit, a valve device which can be switched electrically by the at least one electronic control unit, wherein in a switched state of the valve device pressure can be built up for the trailer braking system, which pressure brings about braking of the trailer, and a pressure sensor which senses the pressure for the trailer braking system and transmits a corresponding signal to the electronic control unit, wherein the pressure for the trailer braking system can be influenced by the electronic control unit by pulsed switching of the valve device taking into account the pressure, sensed by the pressure sensor, for the trailer braking system.
The invention also relates to a method for controlling a braking system for a utility vehicle which can be coupled pneumatically to a trailer, having at least one electronic control unit, a valve device which can be switched electrically by the at least one electronic control unit, wherein in a switched state of the valve device pressure can be built up for the trailer braking system, which pressure brings about braking of the trailer, and a pressure sensor which senses the pressure for the trailer braking system and transmits a corresponding signal to the electronic control unit, wherein the pressure for the trailer braking system is influenced by the electronic control unit by pulsed switching of the valve device taking into account the pressure, sensed by the pressure sensor, for the trailer braking system.
In conjunction with utility vehicles which can be coupled to a trailer, it is known to stabilize the driving behavior by means of what is referred to as an anti-jackknifing braking function. The anti-jackknifing braking function prevents the trailer reaching a higher speed than the traction vehicle on sections of road with a negative gradient, since such a situation could ultimately lead to jackknifing of the vehicle combination and therefore to a very unsafe driving situation. Such an anti-jackknifing braking function can be implemented, in particular, by pulsed actuation of valves which influence the brake pressure of the trailer braking system. The pulsed actuation can, for example, be carried out in a frequency-modulated fashion and, in particular in the case of constant frequency, in a pulse-width-modulated fashion. The actuated valves may be, in particular, switching valves which are primarily provided for switching over between a ventilating state and a venting state, that is to say which are in principle not intended to make available a metered pressure supply. Quasi-continuous metering of pressure in the region of the modulated compressed air can be achieved by virtue of the fact that such switching valves are actuated, for example, in a pulse-width-modulated fashion, that is to say they can be switched over with a certain pulse duty factor between the ventilating state and the venting state. However, it is problematic that pulsed actuation of the switching valves leads to extremely high load change rates, as a result of which the valves are heavily loaded and their service life is restricted.
The invention is based on the object of reducing the loading of switching valves in conjunction with an anti-jackknifing braking function made available by means of pulse width modulation, in order to improve the vehicle stability.
The invention is based on the generic braking system by virtue of the fact that the electronic control unit permits the pulsed switching in order to bring about braking of the trailer when the service brake of the traction vehicle is not active only if a condition applies according to which braking of the trailer could be appropriate despite the service brake of the traction vehicle not being active, and such anti-jackknifing braking is requested by a signal generator. The invention therefore makes available a type of filter which ensures that no unnecessary actuation of the switching valves takes place outside actually desired and appropriate anti-jackknifing braking phases. The switching valves are therefore no longer subjected to continuous pulsed actuation whenever the driver of the utility vehicle makes a corresponding request via a signal generator, and this has a positive effect on the service life of the actuated solenoid valves and brakes. The pulsed actuation can lead, for example, in a pulse-width-modulated fashion to variation of the pressure to be influenced, and the invention is mainly explained using the example of pulse width modulation within the scope of the present disclosure without restricting the generality. The above-mentioned pressure for the trailer braking system may be a control pressure on the basis of which the actual brake pressure for the trailer braking system is generated.
In particular it is possible to provide that the condition corresponds to the presence of at least one of the following states: the vehicle speed is lower than a threshold value, the accelerator pedal is not activated, the engine is braking, the engine is not generating any usable torque, a retarder system is active, the vehicle is located on a section of road with a negative gradient. Such a driving state is frequently present during sustained-action braking since the traction vehicle is then braked by the engine and the retarder system while the trailer is not braked. The retarder system and the engine brake are also referred to as sustained-action brakes. They are frequently activated by a separate signal generator or switch, these switches usually being switched in a stepped fashion. Arrangements in which the engine brake can be switched on independently of the retarder system are also known, but in this context there is usually a possibility of exerting combined influence and carrying out resulting adjustment of the systems with respect to one another. There is frequent provision for the sustained-action braking deceleration to be adjusted automatically to the deceleration of a preceding braking operation by a service brake. The signal generator for the anti-jackknifing braking can be provided separately or combined with the signal generator for the engine braking or the retarder braking. It is also possible to equip the manual control unit for an electric parking brake with an additional function with the result that the signal for the initiation of anti-jackknifing braking can be generated in this way. The condition for the approval of the anti-jackknifing braking can be coupled to the characteristic driving states of the vehicle on this basis. In this context it is possible to take into account only one driving state or else also a plurality of driving states, in particular all the specified driving states. It is possible, for example, to prevent pulse-width-modulated actuation of the switching valves, that is to say the solenoid valves, from taking place if the vehicle is moving at a high speed at which an anti-jackknifing braking function should not occur. It is also possible to provide that when the accelerator pedal is actuated an anti-jackknifing braking function is prevented in all cases because it is improbable that the trailer could run into the rear of the traction vehicle in such a state. Furthermore, a criterion for the approval of the anti-jackknifing braking can be that the engine is braking since this characterizes a typical driving situation which makes anti-jackknifing braking appear appropriate or necessary. Furthermore, it is possible to check, in particular by means of the engine control device, whether the engine is generating a usable positive torque. If this is not the case, this is a criterion for permitting the anti-jackknifing braking. Through combined sustained-action braking using a retarder system it is also possible to bring about deceleration of the utility vehicle. If such a retarder system is active, it may be appropriate to permit anti-jackknifing braking. It is also possible to check whether the vehicle is located on a section of road with a negative gradient, in which case this checking is carried out on the basis of an inclination sensor system or of navigation data. If this checking reveals that a negative gradient is not present, the pulse-width-modulated actuation of the switching valves can also be suppressed. During the control of the anti-jackknifing braking function it is also possible to take into account the wheel speeds of the traction vehicle and trailer as well as the steering angle. It is possible, for example on the basis of a slip comparison between the trailer and the traction vehicle, to determine that the trailer is about to run into the rear of the traction vehicle. Taking into account the steering angle may be appropriate since at different steering angles the vehicle is influenced by anti-jackknifing braking in different ways. With respect to the driving behavior of the utility vehicle it may also be appropriate to take into account the different load distribution, brought about by an e differential, on a sustained-action braking operation which is brought about, in particular, by the engine and a retarder brake. An e differential is a stepless differential for adapting engine acceleration torques to the forces which can be applied to the underlying surface by the wheels. An e differential distributes the wheel forces individually along the various wheels in an appropriate way. Subsequent to a sustained-action braking operation it is appropriate, when the vehicle accelerates due to the accelerator pedal being actuated, to extend the vehicle combination as quickly as possible and not permit the engine speed to drop into a range in which the engine could stall under certain circumstances. In this context it may be desired to release the anti-jackknifing brake before the transition to renewed acceleration of the vehicle combination, which reduces the overall loading on components. Further consideration can also be given to the travel sensor system of the chassis control system and of the pneumatic suspension, which, together with the steering angle and the slip during cornering, provide a point of reference via the shearing forces of the trailer when the sustained-action braking system is activated. Modern vehicles also frequently have an active steering system which can make available steering angle signals to the control electronics during the anti-jackknifing braking process in order to perform an automatic steering angle correction. On this basis, an anti-jackknifing braking process can be ended early in order in this way also to reduce the loading on the components and to improve the driving stability. Furthermore, if an active shock absorber control system is present, it may be helpful to adjust the shock absorber to a higher degree of hardness in order to prevent rolling movements and also in this way to be able to end an anti-jackknifing braking process more quickly, and this also reduces loading on the controlling components, that is to say the valves and the brakes, and improves the driving stability.
There is advantageously provision that a pressure limiter is connected upstream of the valve device. A pressure limiter is appropriate since the number of necessary switching cycles also rises owing to the requirement for more frequent and more sensitive clocking as the supply pressure increases. The pressure limiter therefore has an advantageous influence on the service life of the valves.
It is possible to provide that the valve device is a 3/2-way valve. On this basis, it is possible to implement a venting and ventilation function with a single valve.
It is also possible to provide that the valve device comprises two 2/2-way valves. One of the 2/2-way valves is then used for venting, while ventilation is carried out with the other. If neither of the valves is actuated, the pressure is maintained, in which case ventilation and venting is carried out very frequently during anti-jackknifing braking owing to the critical driving situation.
It is possible for the valve device to make the control pressure available directly. Such a valve device which is configured as a solenoid valve must make available a sufficient flow cross section.
It is also conceivable for the valve device to actuate a relay valve which makes the control pressure available. It is advantageous for the valve device which is actuated in a pulse-width-modulated fashion to make the control pressure available indirectly via a relay valve since the switching valves which are actuated in a pulse-width-modulated fashion can then be dimensioned and configured independently of the control pressure which is necessary for the anti-jackknifing braking. The pressure pulses are integrated in the control chamber of the relay valve to form a median pressure which is then fed as a control pressure preset value to the trailer braking system.
According to one preferred embodiment of the present invention there is provision for the valve device to be integrated into a trailer control module which controls the trailer braking system and supplies it with compressed air.
However, it is also conceivable for the valve device to be integrated into an electropneumatic control unit which pneumatically controls the parking brake of the traction vehicle and the trailer braking system via a trailer control module.
In this context it is then advantageous that at least one throttle which, in the event of a reduction in pressure in the region of a control inlet of the trailer control module, delays a reduction in pressure in a spring-loaded cylinder, is provided in a line system of the electropneumatic control unit, which line system is connected to the spring-loaded cylinder of the parking brake of the traction vehicle and to the control inlet of the trailer control module. Since the electropneumatic control unit couples both the control inlet of the trailer control module and the parking brake cylinders to the compressed air supply, a loss of pressure in the region of the trailer control module could bring about a sudden collapse of the pressure in all the lines of the electropneumatic control unit which are connected thereto, and therefore also in the region of the spring-loaded cylinder. This would result in the traction vehicle being suddenly braked by the parking brake. This is prevented by slowing down a reduction in pressure in the region of the spring-loaded cylinders by means of throttling.
Taking this as a basis, it is then also possible to provide that an unexpected reduction in pressure in the region of the control inlet of the trailer control module can be sensed by the pressure sensor, and the pressure in the spring-loaded cylinder of the parking brake can be held by switching over a valve. The pressure sensor, which is also used in conjunction with the pulse-width-modulated actuation of the valve device, can therefore be employed to sense an unexpected drop in the pressure in the region of the trailer control module and to then disconnect the spring-loaded cylinder from the pressure reducer by switching over a valve device. In combination with the throttling of pressure it is therefore possible firstly to slow down the reduction in pressure in the spring-loaded cylinder independently of the switching over and sensing of pressure, and then to stop it completely, if necessary, by switching over the valve.
It is advantageously possible to provide that a signal which can be generated by a signal generator which can be operated manually can be fed to the at least one electronic control unit, and the anti-jackknifing braking function can be suppressed as a function of the presence of the signal. This provides the driver with the possibility of switching off the anti-jackknifing braking functionality of the utility vehicle from the outset.
It is also possible to provide that a variable signal which can be generated by a signal generator which can be activated manually in different ways can be fed to the at least one electronic control unit, wherein at least one property of the signal depends on the method of activation of the signal generator, and the at least one property of the signal influences the braking behavior of the trailer. In particular, it is possible to provide a signal generator which can be activated in different ways. The pressure which is built up for the trailer brake can therefore be influenced as a function of the signal generator travel.
It is likewise possible to provide that a signal which can be generated by a signal generator which can be activated manually can be fed to the at least one electronic control unit, wherein the activation time of the signal generator influences the braking behavior of the trailer. It is therefore possible, for example, to provide that a relatively high pressure is built up when the signal generator is activated for a relatively long time.
There is also advantageously provision that wheel speed sensors are provided whose signals can be fed to the at least one electronic control unit, and that the vehicle speed which is used in conjunction with the condition for the approval of pulsed switching of the valve device can be derived from signals of the rotational speed sensors. On the basis of a comparison of the trailer and the traction vehicle with respect to slip it is possible to determine, for example, that the trailer is about to run into the rear of the traction vehicle. On the part of the traction vehicle, the tachometer signal or data from a navigation system could additionally be used as correction values. Using active wheel speed sensors permits, in particular at low rotational speeds, the provision of values which can be processed very well, which permits rapid stabilization of the vehicle.
It is also possible to provide that the trailer braking system has disk brakes. This is appropriate, in particular in combination with active wheel speed sensors, since disk brakes permit sensitive pressure reduction and improved meterability with low hysteresis, in particular at low vehicle speeds. In particular in the case of difficult conditions of the underlying surface, for example with chippings (U chippings) it is therefore possible to avoid undesired locking of the wheels.
The invention is based on the generic method in that the electronic control unit permits the pulsed switching in order to bring about braking of the trailer when the service brake of the traction vehicle is not active only if a condition applies according to which braking of the trailer could be appropriate despite the service brake of the traction vehicle not being active, and such anti-jackknifing braking is requested by a signal generator.
In this way, the advantages and particularities of the braking system according to the invention are also implemented within the scope of a method. This also applies to the particularly preferred embodiments of the method according to the invention which are disclosed below.
Said method is advantageously developed by virtue of the fact that the condition corresponds to the presence of at least one of the following states: the vehicle speed is lower than a threshold value, the accelerator pedal is not activated, the engine is braking, the engine is not generating any usable torque, a retarder system is active, the vehicle is located on a section of road with a negative gradient.
Furthermore, it is possible to provide that an unexpected reduction in pressure in the region of the control inlet of the trailer control module is sensed by the pressure sensor, and the pressure in the spring-loaded cylinder of the parking brake is held by switching over a valve.
It is also preferred that a temperature in the region of the trailer braking system is sensed, and when a predefined temperature threshold is exceeded the pressure for the trailer braking system is reduced. This is a safety measure on the basis of which the loading on the trailer braking system and the valves is reduced.
In this context it is also possible to provide that a temperature in the region of the trailer braking system is sensed, and when a predefined temperature threshold is exceeded anti-jackknifing braking is prohibited.
It is possible to take a further safety measure, specifically in that a time during which anti-jackknifing braking is carried out is sensed, and in that when a predefined time threshold is exceeded further anti-jackknifing braking is prohibited. This also prevents possible overloading of the components involved in the anti-jackknifing braking.
It is also possible to provide that during anti-jackknifing braking the activity of the service brake of the traction vehicle is monitored, and that when the service brake of the traction vehicle is activated further anti-jackknifing braking is prohibited. Anti-jackknifing braking in the specific sense is then no longer necessary since the entire braking system of the utility vehicle is activated and it will stop the vehicle combination in an extended state.
It is particularly advantageous that during anti-jackknifing braking the rotational speed of the engine of the utility vehicle is monitored. This monitoring of the rotational speed can be performed, for example, via the engine control device of the utility vehicle, with information relating to this being transmitted to the electronic control unit which is responsible for the anti-jackknifing braking. Such monitoring of the rotational speed is advantageous since in an unfavorable case the anti-jackknifing braking can cause the rotational speed to drop to a range in which the engine can be expected to stall. This should be prevented since then all or virtually all of the functional elements of the vehicle go into a back-up operating mode by changing over, for example, from an electronic control to a pneumatic control, with the result that only a very restricted functionality of the entire braking system is still available. In critical driving situations this adversely affects the maneuverability.
In this context it may be advantageous for the pressure for the trailer braking system to be reduced when a predefined rotational speed threshold is undershot.
Furthermore it is possible to provide that when a predefined rotational speed threshold is undershot the rate of supply of fuel to the engine of the utility vehicle is increased.
As a measure against stalling of the engine it is also possible to provide that when a predefined rotational speed threshold is undershot the charge pressure of a turbocharger is reduced.
Furthermore it is possible that when a predefined rotational speed threshold is undershot compressed air is fed to the engine of the utility vehicle from a compressed air vessel of a compressed air system of the utility vehicle.
Stalling of the engine can also be prevented by a clutch in a drive train of the utility vehicle being at least partially opened when a predefined rotational speed threshold is undershot.
It is also possible to provide that a transmission of the utility vehicle is adjusted to a transmission position with a reduced transmission ratio when a predefined rotational speed threshold is undershot.
As a further measure against stalling of the engine it is possible to provide that the effect of the retarder system of the utility vehicle is reduced when a predefined rotational speed threshold is undershot.
It may also be beneficial for a bypass valve which is arranged between swept volumes of different cylinders of the engine for the purpose of engine braking to be at least partially closed. Such a bypass valve carries out compression work during engine braking by permitting the compressed volume to overflow into the sucking-in piston. Since the bypass valve is closed when there is an excessive drop in rotational speed, the effect of the engine braking can be decreased.
Furthermore it is possible to provide that a throttle valve in an exhaust stream of the engine is adjusted from a throttling position into a less throttling position.
As has already been described in relation to the braking system according to the invention, the method according to the invention can advantageously be developed by virtue of the fact that a signal which is generated by a signal generator which can be operated manually is fed to the at least one electronic control unit, and the anti-jackknifing braking function is suppressed as a function of the presence of the signal.
It is also advantageous that a variable signal which is generated by a signal generator which can be activated manually in different ways is fed to the at least one electronic control unit, wherein at least one property of the signal depends on the method of activation of the signal generator, and the at least one property of the signal influences the braking behavior of the trailer.
The method according to the invention can also be favorably developed by virtue of the fact that a signal which is generated by a signal generator, which can be activated manually is fed to the at least one electronic control unit, wherein the activation time of the signal generator influences the braking behavior of the trailer.
It is also possible for wheel speed sensors to be provided whose signals are fed to the at least one electronic control unit, and for the vehicle speed which is used in conjunction with the condition for the approval of pulsed switching of the valve device to be derived from signals of the rotational speed sensors.
The invention will now be explained by way of example on the basis of particularly preferred embodiments and with reference to the accompanying drawings.